Impedance measuring network

ABSTRACT

A circuit is connected to an ultrasonic transducer being fed by a constant current or constant voltage power supply. By rectifying, integrating, and amplifying the transducer coil voltage an output signal that is a function of the transducer and load impedance is provided that can be displayed so as to determine the operating condition of the ultrasonic apparatus.

United States Patent Wright Dec. 10, 1974 IMPEDANCE MEASURING NETWORK3,212,695 10/1965 MacGregor 228/1 3,445,750 5 1969 Uth t l. [75]Inventor: Loren G. Wright, Alameda County, 3,472,063 loilgg s g i Calif-3,489,241 1 1970 Steinberg .1 228/1 x [73] Assignee: Uthe Technology,Inc., Mountain View, Calif- Primary ExaminerJames J. Gill [22] Filed.June 10, 1971 Attorney, Agent, or FirmLimbach, Limbach &

Sutton 21 App]. No.: 151,965

Related U.S. Application Data ABSTRACT [62] Division of Ser. No.888,330, Dec. 29, l969, Pat. No.

A circuit is connected to an ultrasonic transducer being fed by aconstant current or constant voltage [52 U.S. Cl. 73/67.1 power supply.By rectifying, integrating, and amplify- [51] Int. Cl. GOlm 7/00 ing thetransducer coil voltage an output signal that is Field of sealdl 228/1 afunction of the transducer and load impedance is provided that can bedisplayed so as to determine the [56] References Cited operatingcondition of the ultrasonic apparatus.

UNITED STATES PATENTS 3 Cl 4 D F 3.153 50 10/1964 Worlton.... 29/240'rawmg s IOZ IMPEDANCE MEASURING NETWORK l4 2 l 1 OSCILLATOR PR AMPLIFIERI TRANSDUCER 1 M 1 56 38 I 34 f 1 1 FEEDBACK 4 BIAS REG. BlAS l 1 42 6%SUPPLY SUPPLY 1 I l l l 1 l 46 I 1 I r44 l L MODE I A SWITCH I VOLTAGE Ias PATENTED DEC 10 1974 -BAD WW5 SHEEF 10F 2 M0 CLEAN ICTEON) TIMIMPEDANCE MEASURING NETWORK This application is a division of mycopending application, Ser. No. 888,330, filed Dec. 29, 1969, now US.Pat. No. 3,636,456, issued Jan. 18, 1972. This application is alsorelated to copending application Ser. No. 24,547, filed Apr. '1, 1970and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION This invention relates to an electroniccircuit suitable for use with an ultrasonic power supply of the typedescribed in US. Pat. 3,445,750 to P. M. Uthe, Jr. et a]. In thereferenced patent, a low output impedance power supply is directlyconnected to an ultrasonic transducer of the type used for cleaning,soldering, drilling, bonding, brazing and welding of materials and afeedback loop is provided to correct for changes in the transducer and.load in order to maintain I constant transducer current (for drivingmagnetostrictive transducers) or voltage (fordriving ferroelectric andpiezoelectric transducers).

It has been found that the variations in transducer voltage or currentreflected in the changes in the feedback loop voltage are a measure ofchanges in transducer and load impedance. Further, the manner of theseimpedance variations is uniquely related to such factors as themechanical connection of parts of the bonder such as the horn or amagnet on the transducer or the transducer mounting bracket, the amountof friction in those bearing of the bonder allowing rotation of thetransducer, the electrical connection of the transducer to the powersupply, the hardness of the wire being bonded, the surface conditions ofthe parts being joined and the quality of the bond formed.

It is therefore an object of this invention to provide a signal outputresponsive to the impedance changes of a transducer and load used withthe ultrasonic power supply of the referenced patent. It is a'furtherobject to provide a signal output of a form readily usable by visualdisplay instruments including oscilloscopes and permanent recordingdevices.

SUMMARY OF THE INVENTION An electronic circuit apparatus is provided forsensing the impedance changes in an ultrasonic transducer. The circuitinput is connected to the output of the transducer power supply which isconnected directly to the ultrasonic transducer. A half-wave rectifierand smoothing filter provides a DC signal having a varying amplitudedepending on the impedance variation in the transducer and load. The DCsignal is applied to a differentiating amplifier to provide an output toan external display device. A biasing-type arrangement is provided topermit the output signal to be adjusted for maximum resolution ormagnification on the external display. 1

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side elevationalview showing the general elements used to practice ultrasonic wirebonding where the present invention can be useful.

FIG. 2 is a block diagrammatical view of the power supply for anultrasonic wire bonder incorporating the present invention.

FIG. 3 is a schematic circuit diagram of a preferred embodiment of theimpedance measuring network according to the instant invention.

FIG. 4 is a graph showing typical traces on a display device utilizingthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the present invention isdirected to an impedance measuring network useful with many differentdevices and with many different applications, the network isparticularly suited for use with an ultrasonic transducer for providingwelds between miniature members such as wire bonding contacts ofintegrated circuits. Additionally, the present invention includescertain aspects particularly related to ultrasonic wire bondingtechniques and accordingly the invention will be described for purposesof full disclosure as 'well as purposes of illustration as practicedwith apparatus for ultrasonic wire bonding.

Referring now to the drawing with particular reference to FIG. 1, thereis shown a schematic side elevational view of apparatus useful inultrasonic wire bonding. The apparatus includes a base or support frame11 on which is located a mounting yoke 12 pivotally connected to ahollowcylindrical clamping member 13 within which a transducer 14 is held byway of a collar 15 onto which the clamp is tightened, such as by meansof a screw or bolt 16. The transducer 14 is any one of a variety ofstructures for converting electrical energy into high frequency motion.The transducer 14 includes in or on its body an active element 17 whichis typically either magnetostrictive or electrostrictive. High frequencyelectrical energy such as in the range of 60 kHz connected to the activeelement 17 via lead 18 from a power supply l9 results in vibration ofthe transducer body. The shape, size and composition of the transducerdetermine the mechanical resonant frequency thereof, and a power supplycapable of delivering a frequency equal to the resonant mechanicalfrequency is employed to drive the transducer in the performance of itsintended function.

The vibrational energy of the transducer is concentrated at one end viaan exponential horn 21, typically connected to the transducer body by ascrew 22 and provided at its free end with an element for coupling thevibrational energy to the work.

In the case of wire bonding the vibrational energy is directed in aplane parallel to the surface of the elements to be joined, andaccordingly the member for coupling energy from the born to the work isa wedge 23 supported in a transverse bore through of the horn 21 andheld in place via a set screw 24.

The workpieces are supported beneath the wedge 23 from an anvil 25 ontowhich the lower of the pieces 26 such as an integrated circuit chipsupport or transistor can is clamped such as by a chuck 27. The othermember 28 to be joined to the piece part 26 is located between the lowerend of wedge 23 and the piece part 26. In the case of wire bonding thepart 28 is a wire on the order of 0.0005 to 0.001 inches which is fedfrom a spool (not shown) into bonding location. For accurate location ofsuch a fine wire a small aperture is often provided in the heel portionof the wedge so that the wire can be threaded therethrough into locationbeneath the forward portion of the wedge.

A weight 29 is providedonthe transducer mounting assembly to produce thedesired amount of static force to hold the work members together duringthe bonding operation.

A bond is accomplished typically by rotating the transducer 14 about thepivot 12' to drop the lower edge of the wedge 23 onto the workpieces.Then, the high frequency signal is applied from the power supply to theactive element 17 for a very short period of time so that a bondresults.

Depending upon the size, shape and composition of the elements betweenthe active member 17 and the surface where a bond is to be createdbetween parts 26 and 28, there are certain frequencies of mechanicalresonance at which the assembly can be excited at large amplitudes witha sinusoidal electrical signal input. Nu merous changes in thetransducer and coupling elements can result in a change in impedancepresented to the power supply 19. The present invention provides animpedance measuring network which reflects these changes of impedancefor apprising the operator of the apparatus of the operational conditionof both the apparatus and the performance of the bonding operation. Amemory oscilloscope can be used to determine performance.

A power supply useful in ultrasonic wire bonding wherein changes ofimpedance do occur is one in which the power delivered to the transduceris controlled in accordancewith these changes in impedance. Such a powersupply is described in US. Pat. No. 3,445,750 referred to above.

The impedance measuring network of the present invention connected tothe power supply of Pat. No. 3,445,750 provides the appropriate displayfor the bonder operator in accordance with the aspects of thisinvention.

Referring now to FIG. 2, there is shown an ultrasonic power supply 19 ofthe type usable in connection with the present invention. An oscillator34 generates a sine wave having a frequency substantially equal to theresonant frequency of the transducer 14 and load. A signal from feedbackcircuit 42 along with the output of oscillator 34 are applied topreamplifier 36 and to an amplifier 38. Precise regulation of theamplifier 38 is provided by a bias supply 68. A mode switch 44 operatesin conjunction with feedback circuit 42 and a fixed regulated biassupply46 to provide either a constant current or a constant voltage totransducer 14, depending on the type of transducer used, eithermagnetostrictive or electrostrictive, respectively. Bias supply 46provides a fixed DC bias for magnetostrictive transducers. A powersupply of this type is explained in greater detail in the referencedpatent.

An impedance measuring network 102 according to this invention isconnected across the transducer 14. Referring now to FIG. 3, the detailsof network 102 are shown. Network 102 is connected to terminals 50 and52 of an ultrasonic power supply. A transducer coil 104 is connected toterminals 50 and 52. A low valued resistor 55 provides a voltageproportional to current flow through coil 104 when the power supplyoperates in the constant current output mode. Thus a voltage input toimpedance network 102 is always presented at terminals 50-52.

The signal applied to network 102 is half wave rectified by a diode 106connected with its anode to terminal 50 so that positive pulses areprovided at its cathode for smoothing in a filter. network 107consisting of a capacitor 108 connected from the anode of diode 106 to ajunction 109 connected to terminal 52, a resistor 110 connected from theanode of diode 106 to a junction 111, a capacitor 112 connected fromjunction 111 to junction 109, a resistor 114 connected across capacitor112, a resistor 116 connected between junction 111 and a junction 117, acapacitor 118 connected between junction 117 and junction 109, and aresistor 120 connected between junction 117 and a junction 121. A pairof back-to-back Zener diodes 122 and 124 are connected between junctions121 and 109 in order to limit the voltage output from the filter.Capacitors 108, 112 and 118 are electrolytic capacitors of the typenormally used in power supply applications.

The Zener limited filter output at junction 121 is applied to thenegative input 146 of an operational amplifier (op. amp.) 142. Thepositive input 148 is taken from junction 109. Capacitor 144 isconnected across terminals 149-151 to stabilize the op. amp. to preventself-oscillation. Amplifier 142 also provides gain necessary to drivethe external display-device and in addition provides isolation from theultrasonic power supply.

A potentiometer connected between op. amp. terminals 140 and 154 adjuststhe amplifier gain. The network 102 output is taken at terminal 154. ADC supply source suchas batteries 134 and 136 provide a positive andnegative power supply to terminals 150 and 152 of op. amp. 142,respectively. DPST switch 138 switches the voltages to turn theamplifier on and off.

When the switch 138 is on, a negative voltage is also applied to a SPSTbias switch 132 and to an end of a potentiometer 130. The rider of pot130 is connected to one end of a pot 128 which has its other endconnected to junction 109. The rider of pot 128 is connected throughresistor 126 to junction 117. This bias voltage allows the output signalfrom network 102 to be adjusted so that only a portion of the totalwaveform is presented. By doing this, for example, the peak of awaveform may be looked at with greater magnification on a displaydevice.

In operation, when the current or voltage through transducer coil 104remains constant, a, constant amplitude sine wave voltage is applied tonetwork 102. The inputs to op. amp. 142 are thus steady DC voltages thatare compared to provide a zero output at terminal 154. However, as theimpedance of the transducer and load changes, the sine wave will changethus causing the input to op. amp. 142 to vary from the former steadystate DC level. The variation is differentially compared to provide anoutput at terminal 154. The structure of the waveform at terminal 154has been found to provideinformation as to the nature of the ultrasonicprocess being performed.

Such a waveform is shown in FIG. 4 of the drawing wherein the solidcurve shows the typical waveform presented by the impedancemeasuringnetwork of this invention while an ultrasonic bond isaccomplished with this invention. lt will be noted that the impedanceincreases with time until the actual bond is achieved.

Variations in curves from the typical curve or is preventingreproducibility for accomplishing consistently good welds.

The checkout procedure to locate the trouble can follow the followingformat. First of all with the transducer 14 in elevated position asshown in FIG. 1 and not in contact with the members to be joined signalsfrom power supply 19 are applied to the transducer and traces aredisplayed for successive timing cycles. Failure to achievereproducibility under these circumstances typically indicates thatsomething is loose in or around the transducer 14. This may be a horn 21that is not tightly screwed onto the transducer body, a loose wedge 23,a loose magnet in the active element section 17, a loose clamp 13 orpossibly friction on the bearings at the pivot 12'.

Once reproducibility is achieved with the transducer in elevatedposition it can be dropped to bring the end of the wedge 23 in contactwith the work. Failure to achieve reproducibility in this position canindicate nonuniform friction on the bearings at pivot 12', a loose clampin the chuck 27 or nonuniform characteristics such as grease on thesurface of the work.

Next, the aspects of the weld and welded members can be checked byactually performing welding operations. Failure to achievereproducibility in accomplishing welds can indicate changes in thehardness of the wire being used or failure to accomplish a good weld byreason of failure of the wedge to sufficiently grip the wire,insufficient or too much time for the application of the ultrasonicvibrations to the weld, or the like.

Thus it will be .seen that the present invention provides a qualitycontrol for performing ultrasonic welds as well as a method andapparatus for checking out malfunctions in ultrasonic welding machines.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it is understood that certain changes and modificationsmay be practiced within the spirit of the invention as limited only bythe scope of the appended claims.

I claim:

1. Apparatus for driving and testing the operation of an ultrasonictransducer comprising a constant current or constant voltage powersupply for driving an ultrasonic transducer for application to a load, Icircuit means for rectifying, integrating and amplify ing transducercoil voltage as a signal that is an analog of the transducer and loadimpedance, and means for displaying said signal.

2. Apparatus for driving and testing the operation of an ultrasonicwelding system including a transducer acting on a load, said transducerhaving a coil across which a voltage appears during operation of thesystem, comprising a constant current or constant voltage power supplyfor driving said transducer, and

means for generating a waveform signal that is a function of thetransducer and load impedance, said means comprising circuit means forrectifying, integrating and amplifying said transducer coil voltage toprovide a waveform signal that is an analog of the transducer and loadimpedance.

3. Apparatus for driving and testing the operation of an ultrasonicwelding system including a transducer acting on a load, said transducerhaving a coil across which a voltage appears during operation of thesystem, comprising a constant current or constant voltage power supplyfor driving said transducer,

means for generating a waveform signal that is a function of thetransducer and load impedance, said means comprising circuit means forrectifying, integrating and amplifying said transducer coil voltage toprovide a waveform signal that is an analog of the transducer and loadimpedance, analyzing means for displaying an applied signal, and

means for applying said waveform signal to said analyzing means so thatan operator may determine the operating condition of the system.

1. Apparatus for driving and testing the operation of an ultrasonictransducer comprising a constant current or constant voltage powersupply for driving an ultrasonic transducer for application to a load,circuit means for rectifying, integrating and amplifying transducer coilvoltage as a signal that is an analog of the transducer and loadimpedance, and means for displaying said signal.
 2. Apparatus fordriving and testing the operation of an ultrasonic welding systemincluding a transducer acting on a load, said transducer having a coilacross which a voltage appears during operation of the system,comprising a constant current or constant voltage power supply fordriving said transducer, and means for generating a waveform signal thatis a function of the transducer and load impedance, said meanscomprising circuit means for rectifying, integrating and amplifying saidtransducer coil voltage to provide a waveform signal that is an analogof the transducer and load impedance.
 3. Apparatus for driving andtesting the operation of an ultrasonic welding system including atransducer acting on a load, said transducer having a coil across whicha voltage appears during operation of the system, comprising a constantcurrent or constant voltage power supply for driving said transducer,means for generating a waveform signal that is a function of thetransducer and load impedance, said means comprising circuit means forrectifying, integrating and amplifying said transducer coil voltage toprovide a waveform signal that is an analog of the transducer and loadimpedance, analyzing means for displaying an applied signal, and meansfor applying said waveform signal to said analyzing means so that anoperator may determine the operating condition of the system.